A disc drive is controlled by an embedded microprocessor that executes software instructions stored on more than one memory device. (Herein, software and firmware are referred to jointly as “software.”) Typically, the software is divided into modules dedicated to performing certain sorts of operations. For example, a software module may contain various functions, all of which are dedicated to driving the servo system of the disc drive. By virtue of the role to which each software module is dedicated, each such module has a different requirement with respect to execution speed. Certain software modules need to be executed rapidly, because they relate to time-critical functions. Other software modules do not need to be executed as quickly.
One manner of ensuring that time-critical software modules are executed quickly is to store each such module in a rapid-access memory unit. If this can be done, then the microprocessor is able to retrieve the software instructions quickly, and is able to execute them promptly. Thus, per such a scheme, time-critical software modules are stored in a rapid-access memory unit, while non-time-critical software modules are stored in a less expensive slow-access memory unit.
As drive technology has advanced, there has been a corresponding increase in the number of commands a disc drive is required to execute. Therefore, the size of the various time-critical software modules has expanded. As a consequence of this size expansion, disc drive manufacturers are confronted with a dilemma. It is no longer possible to store all of the time-critical software modules in the rapid-access memory unit. Thus, either the rapid-access memory unit needs to be expanded or one of the time-critical software modules needs to be stored in a slow-access memory unit. Each approach has drawbacks. Increasing the amount of available rapid-access memory is costly and puts the disc drive manufacturer at a competitive disadvantage. On the other hand, storing a time-critical software module in a slow-access memory unit, while inexpensive, is harmful to the performance of the disc drive.
As is evident from the foregoing discussion, there exists a need for a scheme by which a disc drive may virtually expand the size of its rapid-access memory space. A desirable solution will accomplish this end with little or no additional cost to the disc drive manufacturer.